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Introduction to the 8255 PPI
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Today, we will learn about the Intel 8255 Programmable Peripheral Interface, commonly known as PPI. Can anyone tell me why peripheral interfaces are important in microprocessor systems?
They help the CPU communicate with external devices, right?
Exactly! The 8255 PPI allows the 8085 microprocessor to interact with devices like keyboards, displays, and printers. What do you think are some advantages of using parallel I/O?
It can send multiple bits of data at once, which is faster than serial I/O.
Correct! High data transfer rates are possible with parallel communication, especially for devices that can handle multiple data lines simultaneously.
So, how does the 8255 actually work?
Great question! The 8255 has various functional blocks that manage data transmission and control signals. Would anyone like to name one of these blocks?
The Data Bus Buffer!
Yes! The Data Bus Buffer interfaces directly with the microprocessor’s data bus, allowing for bidirectional data flow.
To summarize, the PPI is key for facilitating efficient communication between the CPU and peripherals, particularly through its programmable features and high-speed capabilities.
Functional Blocks of 8255
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Let’s dive deeper into the internal structure of the 8255. Who can tell me why we have a Read/Write Control Logic block?
I think it manages the read and write operations between the CPU and the 8255.
Exactly! This block helps control when data gets written to or read from the internal registers. Now, what about the Group A and Group B Controls?
They configure the functionality of Ports A and B, right?
Correct! Group A Control manages Port A and the upper bits of Port C, while Group B Control oversees Port B and the lower bits of Port C. Why might we want to configure Ports A and B differently?
We might want one as input and the other for output, depending on what we’re connecting.
Exactly! Understanding these blocks is vital for effective interfacing. As a recap, the distinct functional blocks of the 8255 allow for organized control over the data flow and peripheral connection.
Operation Modes of 8255
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Now, let’s explore the operational modes of the 8255. Can someone share what Mode 0 is?
Mode 0 is Basic I/O, where all ports can be used simply for input or output.
Exactly! It’s straightforward and widely used. How does this differ from Mode 1?
Mode 1 uses handshaking for data transfer; it’s meant for more reliable communication.
Well said! Handshaking is crucial in applications where data integrity is necessary. Lastly, what about Mode 2?
Mode 2 allows Port A to function bidirectionally, enabling it to send and receive data at the same time.
Exactly! Mode 2’s bidirectional ability makes it versatile for complex interactions. To wrap up this session, we’ve concluded that understanding the operational modes gives us flexibility in peripheral interfacing.
Control Word Format
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Next, we’ll review how we configure the 8255 using a control word. What’s the significance of the D7 bit in the control word?
It indicates whether we are setting the device to I/O mode or BSR mode.
Correct! D7 being '1' sets the control word for I/O mode, and '0' sets it for BSR mode. Can someone tell me what we can configure with the bits D6 and D5?
Those select the operational mode for Group A.
Exactly! The combination of these bits creates a specific control word that directs how the 8255 will function. Why do you think it’s important to configure the control word before using the ports?
Because it sets up the ports for their intended use, enabling proper communication!
Well answered! Without this initial configuration, the ports wouldn’t function correctly.
Interfacing the 8255 with 8085
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Finally, let’s discuss the interfacing of the 8255 with the 8085 microprocessor. Why is proper connection of data and control lines crucial?
Because it ensures that the data is correctly transmitted and received between them.
Exactly! The connections are fundamental for the entire communication process. Can anyone tell me how the chip select (CS) functionality operates?
CS needs to be activated to allow the microprocessor to communicate with the 8255.
Correct! The CS signal is vital for enabling the interface. After connecting the control lines, why is address decoding necessary?
To ensure that the 8085 knows which device it’s trying to communicate with when multiple devices are connected.
Great job! So, as we conclude, remember that proper interfacing is key for smooth and effective communication in microprocessor systems.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
The Intel 8255 PPI acts as a programmable peripheral interface, allowing for efficient parallel Input/Output interactions between the microprocessor and various peripheral devices. The section covers its functional blocks, operational modes, control word format, pin descriptions, and the interfacing process with the 8085 microprocessor.
Detailed
The Intel 8255 Programmable Peripheral Interface (PPI) is integral in enabling microprocessors, like the 8085, to efficiently manage parallel I/O operations with peripherals. The 8255 features 24 programmable I/O pins that can be customized via software to operate in different modes based on application requirements. This includes three operational modes for data communication: Mode 0 (Basic I/O), Mode 1 (Strobed I/O), and Mode 2 (Bidirectional I/O). Each of the ports A, B, and C can be selectively configured for input or output, allowing robust data exchange. Important controls, such as the Data Bus Buffer, Read/Write Control Logic, and specific groups controlling Port C functionality, enable this interaction. Understanding the pin descriptions and control word format is crucial for proper configuration. The process of interfacing the 8255 with the 8085 microprocessor includes setting up connections to data lines, control lines, and address lines through address decoding logic. This section sets the foundation for advanced microcontroller applications by demonstrating the principles of peripheral interfacing.
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Overview of the 8255 PPI
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The 8255 is a versatile, programmable peripheral interface device designed to interface microprocessors with parallel I/O devices. It provides 24 programmable I/O pins, which can be configured by software in various modes to suit different application requirements.
Detailed Explanation
The Intel 8255 PPI is a crucial component in connecting a microprocessor, like the 8085, with external devices. It has a total of 24 pins that can be programmed to perform either input or output operations. This flexibility allows it to be adapted for various applications based on user needs. For instance, if you need to read data from multiple sensors, you can configure the pins as inputs. Conversely, if you want to control motors or lights, you can set them up as outputs.
Examples & Analogies
Think of the 8255 PPI as a smart control panel in a factory. Just like an operator can adjust various controls to operate machines, the 8255 allows a programmer to change the configuration of its pins via software, deciding which devices will receive data or send data based on the situation.
Functional Blocks of the 8255
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The 8255 PPI consists of the following functional blocks:
● Data Bus Buffer: This is a tristate 8-bit bidirectional buffer that interfaces the 8255 with the system data bus (D0-D7 of 8085). It allows the CPU to read data from or write data to the 8255's internal registers (Port A, Port B, Port C, or Control Word Register).
● Read/Write Control Logic: This block manages the internal read and write operations. It accepts control signals from the microprocessor (RD, WR, A0, A1, CS) and generates appropriate internal control signals for the 8255's various functional units.
● Group A Control: This block controls the functionality of Port A and the upper 4 bits of Port C (PC4-PC7). It handles the configuration of these ports based on the control word written by the CPU.
● Group B Control: This block controls the functionality of Port B and the lower 4 bits of Port C (PC0-PC3). It configures these ports based on the control word.
● Port A (PA0-PA7): An 8-bit I/O port, programmable as either an 8-bit input or an 8-bit output. It can operate in Mode 0, Mode 1, or Mode 2.
● Port B (PB0-PB7): An 8-bit I/O port, programmable as either an 8-bit input or an 8-bit output. It can operate in Mode 0 or Mode 1.
● Port C (PC0-PC7): An 8-bit I/O port, which can be divided into two 4-bit nibbles: Port C Upper (PC4-PC7) and Port C Lower (PC0-PC3). Port C can be configured for input or output in Mode 0 or its bits can be individually set or reset in Bit Set/Reset (BSR) mode. It also serves for handshaking signals in Mode 1 and Mode 2.
Detailed Explanation
The 8255 PPI is built from several functional blocks that each serve a unique purpose. Each port (A, B, and C) has its own specific function that can be configured through software:
- Data Bus Buffer: Think of this as a translator that converts data between the CPU and the 8255; it allows data to flow in and out.
- Read/Write Control Logic: This acts like a traffic manager, ensuring the data flows when it's supposed to, according to the signals received from the CPU.
- Group A and Group B Control: These blocks control the groupings of ports and how they function based on the commands given. This means that the CPU can quickly change how these ports behave without additional hardware changes.
- Ports A, B, and C: Each port can be individually programmed to be an input or output, giving great flexibility to the system. Port C even allows us to mix and match configurations to utilize the same port for multiple purposes, enhancing overall system efficiency.
Examples & Analogies
Imagine a traffic intersection where certain roads are open or closed depending on the time of day. Here, the Data Bus Buffer is like traffic lights that allow cars (data) to flow into specific roads (ports). The Read/Write Control Logic acts like a traffic cop, ensuring that cars only go through when it's safe, and the Group controls are like signs that direct this traffic based on preset patterns.
Pin Description of the 8255
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● D0-D7 (Data Bus): 8-bit bidirectional data lines for communication with the microprocessor.
● A0, A1 (Register Select): These are input pins connected to the microprocessor's lower address lines (A0, A1). Along with the Chip Select (CS) signal, they select one of the 8255's internal registers for CPU access:
- A1 A0 | Selection
- ----- | -----------
- 0 0 | Port A
- 0 1 | Port B
- 1 0 | Port C
- 1 1 | Control Word Register
● CS (Chip Select): An active-low input. When CS is low, the 8255 is enabled for communication with the CPU. If high, the 8255 is disabled, and its data bus pins (D0-D7) are in a high-impedance state.
● RD (Read): An active-low input. When CS is low and RD is low, the 8085 reads data from the selected port or internal register.
● WR (Write): An active-low input. When CS is low and WR is low, the 8085 writes data to the selected port or internal register.
● RESET: An active-high input. When high, it clears all internal registers, sets all ports to input mode, and clears the control word. It should be connected to the 8085's RESET OUT.
● Vcc: +5V power supply.
● GND: Ground reference.
● PA0-PA7: 8-bit I/O lines for Port A.
● PB0-PB7: 8-bit I/O lines for Port B.
● PC0-PC7: 8-bit I/O lines for Port C.
Detailed Explanation
The pins of the 8255 chip play significant roles in its operation:
- Data Bus (D0-D7): These are used to send data between the 8255 and the CPU, like how lanes on a highway allow for the flow of vehicles.
- Register Select (A0, A1): These pins tell the 8255 which specific register (Port A, B, or C, or the Control Word Register) should be accessed. It's like selecting a specific department in a large store.
- Chip Select (CS): This pin activates or deactivates the 8255 for communication, ensuring it only processes signals when needed, much like a gate that opens only for authorized vehicles.
- Read (RD) and Write (WR): These pins determine whether the CPU is reading data from or writing data to the 8255. Imagine these as the back and forth signals of walkie-talkies indicating when to talk or listen.
- RESET: This pin clears all previous configurations and prepares the 8255 for a fresh start, akin to resetting a game console before starting a new game.
Examples & Analogies
Picture the structure of a busy office. The data bus (D0-D7) represents the hallway where employees (data) walk in and out. The register select pins (A0, A1) are like signs indicating which room (register) the employees need to go to. The chip select (CS) functions like a security guard allowing employees in only when necessary. Read and write pins are analogous to an open door for a meeting where actions either take place (reading or writing) based on who is inside, and the reset pin is like when the office is cleared after hours and reorganized before a new day.
8255 Modes of Operation
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The 8255 can operate in two primary modes:
1. Bit Set/Reset (BSR) Mode: Only applicable to Port C. Allows individual bits of Port C to be set (high) or reset (low) without affecting other bits.
2. I/O Mode: Configures Ports A, B, and C as input or output ports. This mode has three sub-modes:
- Mode 0 (Basic I/O): All ports (A, B, C) can be configured as simple latched outputs or buffered inputs. No handshaking signals are used.
This is the simplest mode and most common for basic parallel I/O.
- Mode 1 (Strobed I/O): Used for data transfer with handshaking signals. Port A and Port B use Port C lines for handshaking.
- Mode 2 (Bidirectional I/O): Only Port A can be configured in this mode. It allows Port A to be used for both transmitting and receiving data simultaneously, with handshaking.
Detailed Explanation
The 8255 supports two main modes that allow it to operate flexibly based on the application requirements:
1. Bit Set/Reset (BSR) Mode: This mode specifically allows individual bits of Port C to be manipulated. This means you could decide to turn on one light without disturbing the others connected to the same port.
2. I/O Mode: This more commonly used mode formats the ports for either input (reading data) or output (sending data) operations. There are three sub-modes:
- Mode 0: Useful for simple tasks where signals just need to be sent or received without any complex procedures.
- Mode 1: This mode incorporates additional features for handshaking during data transfers, which is crucial for ensuring data integrity.
- Mode 2: This is a more advanced configuration allowing simultaneous data transmission and reception, which is essential when both sending and receiving signals at the same time, like a conversation over a phone.
Examples & Analogies
Imagine a classroom where students (data) can either ask questions or provide answers depending on the mode of communication:
- In BSR mode, one student can raise a finger to answer, while others remain quiet. It's targeted and discrete.
- In I/O mode, if it's a question and answer session, students might either ask questions (input) or provide answers (output). In Mode 0, responses are straightforward; in Mode 1, there's a back-and-forth communication protocol, and in Mode 2, students might be engaging in discussions simultaneously rather than just waiting for turns.
Control Word Format for 8255
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The functionality of the 8255 is configured by writing a specific 8-bit Control Word to its Control Word Register (CWR). The format of this word depends on whether you are setting up I/O modes or using the Bit Set/Reset feature.
A. I/O Mode Set Control Word (D7 = 1):
This control word is written to address 83H (or corresponding CWR address) to configure the modes and directions of Ports A, B, and C...
B. Bit Set/Reset (BSR) Mode Control Word (D7 = 0):
This control word is used to individually set or reset any one of the 8 bits of Port C. It is also written to the Control Word Register (e.g., address 83H).
Detailed Explanation
The Control Word plays a crucial role in determining how the 8255 operates. This word is an 8-bit command sent to the Control Word Register:
- For I/O Mode, the configuration determines how each port behaves. The bits in the control word define whether the ports are inputs or outputs and which modes they are set to. For instance, setting D7 to '1' indicates we’re using I/O modes.
- For the BSR Mode, the control word structure is designed differently, focusing on allowing the user to set or reset specific bits without disturbing others. In this case, D7 is '0' to indicate the BSR mode.
Each bit has a specific action and needs to be carefully set to reflect the intended operations for the ports. Understanding the bit layout helps ensure the device behaves as required.
Examples & Analogies
Consider the Control Word like a recipe for making a dish. Just as specific ingredients (bits) determine the outcome of the meal, the specific bits in a control word dictate how the 8255 operates. If you want to make spaghetti (I/O mode), you need to gather the right ingredients: noodles (input/output settings). If you’re just making a sauce (BSR mode), you only focus on that specific part, adjusting it without influencing the entire dish.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Intel 8255: A programmable device for managing parallel I/O operations with microprocessors.
-
Control Word: An 8-bit configuration that dictates the operational modes and port directions.
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I/O Modes: Various operating modes (Mode 0, Mode 1, Mode 2) for configuring port functionality.
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Pin Description: Each pin of the 8255 has specific functions like data input/output and control.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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Example of writing a control word to set Port A as output, Port B as input, and resetting the control word after a system reset.
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Examples of applications where 8255 enables LED control and switch reading.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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For data transmission, don't delay, 8255 leads the way!
📖 Fascinating Stories
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Picture a post office (the 8255) delivering letters (data) to many houses (ports). Each house can either send or receive mail depending on the control word's instructions.
🧠 Other Memory Gems
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Remember 'PICS' for the 8255 functionality: Ports, Inputs, Control, and Selection.
🎯 Super Acronyms
Use 'MICE' for remembering the modes
- Mode 0
- Input/Output
- Control
- Enable.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
-
Term: PPI
Definition:
Programmable Peripheral Interface - A device that allows microprocessors to control and communicate with peripheral equipment.
-
Term: Control Word
Definition:
An 8-bit word written to the control register used to configure how the 8255 operates.
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Term: I/O Modes
Definition:
The various modes of operation (Mode 0, Mode 1, Mode 2) that define how the ports can be used.
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Term: Bit Set/Reset (BSR) Mode
Definition:
A mode that allows individual bits of a port to be set or reset without affecting others.
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Term: Handshaking
Definition:
A process that ensures data integrity in communication between devices.